Put a male and female bird-of-paradise side by side and it’s always the bright, glittering, fluttering, posing male that grabs our attention. But the females are just as important—if not more so—in determining what birds-of-paradise look like. Here’s why:

1. It started with Darwin

Female choice is a form of sexual selection—Charles Darwin’s “other big idea,” after natural selection. For years, Darwin wrestled to understand how ostentatious display traits—like a bird-of-paradise's plumes or a peacock's tail—could improve the survival of an individual. Eventually he grew so frustrated that he wrote to a friend, "The sight of a feather in a peacock's tail, whenever I gaze at it, makes me sick!" But finally he hit on the elegant solution of sexual selection: traits that improve an individual's mating success can be selected by female choice the same way as traits that improve survival. Sexual selection by female choice offered a solution to the otherwise baffling evolution of beauty in nature.

2. It puts females in the evolutionary drivers seat

In birds-of-paradise, males do the showing-off and females do the choosing. This makes females into “arbiters of evolution"—they shape the course of evolution through their choices. Over millions of years, their preferences for what's attractive have trumped most other factors influencing how males look and act. Beauty has trumped flying ability, inconspicuousness from predators, thermoregulation, and so on. The male birds-of-paradise we see today are the physical manifestation of many generations of past female preferences.

3. You can watch it happening

Take a look at how intently female birds-of-paradise examine displaying males. It’s pretty apparent that sexual selection by female choice is occurring as these birds crane their necks and nearly fall off their perches to check out males. They’re so choosy that it takes many visits to many males before anything happens. A long-term field study in the 1980s found that female Lawes's Parotias visited displaying males up to six weeks before and up to six weeks after mating! Ed and Tim have seen other female birds-of-paradise spend 5–6 hours a day watching males display. Extreme female choice indeed.

4. It’s a matter of taste, at first

How does an extravagant display begin to evolve? It starts with an innate, possibly random preference in a female for a showy male trait. In some animals, these display traits actually give females information about the relative health and condition (sometimes called “quality”) of potential mates. But evidence is accumulating that this doesn’t always have to be the case. Elaborate display traits can evolve simply because they are attractive—because they satisfy some innate preference that females have. When a female with such a preference chooses a male with a showy trait, their offspring inherit both the showy trait and the preference, reinforcing the pattern.

5. Beauty creates diversity

Female preference for aesthetic traits is one element that has allowed so many strange birds-of-paradise to evolve. Unlike traits that indicate a mate’s “quality,” a simple preference for aesthetics or “beauty” opens up many possibilities. As an analogy, think of fashion items like shoes, phones and phone cases, or purses. Our preferences for them are only partly based on how well they function. They’re largely based on how they appeal to us. If we chose items based on their function (comfort, reliability, etc.), we wouldn’t expect to see the riot of colors, shapes, and styles that we see. All this diversity exists because we judge them as objects of beauty rather than utility. There are nearly infinite ways for them to be beautiful, but many fewer ways for them to be functional. In the same way, as the birds-of-paradise seem to show, there are many ways for animals to be aesthetically attractive.

6. It’s not a level playing field

It may sound cruel, but lots of male birds-of-paradise will never get the chance to mate. This is why female choice is so powerful. Individual choices by females tend to converge on a handful of males—the ones perceived to be the most attractive. As a consequence, those few males father most of the offspring. The genetic makeup of the next generation is heavily influenced by those few chosen males. Similarly, the next generation of females reflects their mothers’ preference for those same male traits. The outcome at the end of a year is that the “genetic score” is lopsided—skewed toward a handful of successful males—and pushes evolution of the display trait a little farther along each time.

7. Females are as extraordinary as males

A male bird-of-paradise may be a squawking, yellow-and-red bird hanging upside down from a tree branch, but the female is a bird that can minutely observe this bizarre scene and make an informed decision. The "biology of female choice" is just as extreme (and extremely interesting) as the biology of male displays—and much less well understood.